Method for providing electroluminescent light from a polycyclic heterocyclic compound in an aprotic solvent under electric current

ABSTRACT

Method for providing electroluminescent emission in an electrolytic cell by subjecting to an alternating current an electrolytic cell containing at least two electrodes in a mixture of (a) a substantially inert solvent, (b) a fluorescent organic polycyclic heterocyclic compound, and a supporting electrolyte.

Maricle et al.

Inventors: Donald Leonard Maricle, Ridgefield;

Michael McKay Rauhut, Norwalk,

both of Calif.

Assignee: American Cyanamid Company,

Stamford, Conn.

Filed: Dec. 13, I971 Appl. No; 211,802

Related US. Application Data Division of Ser. No. 504,11 1, Oct. 23, 1965, Pat. No. 3,654,525, which is a continuation-in-part of Ser. No. 382,408, July 13, 1964, abandoned.

US. Cl 315/246, 313/108 R, 313/358, 204/131 1 June 11, 1974 [51] Int. Cl. 1105b 43/00 [58] Field of Search 313/358, 108 R, 108 A, 3l3/109.5; 315/246 [56] References Cited UNITED STATES PATENTS 3,213,440 10/1965 Gesteland et al. 313/358 X 3,248,588 4/1966 Blazek et al. 313/108 3,319,132 5/1967 Chandross et al. 313/358 X Primary Examiner-Palmer C. Demeo Attorney, Agent, or Firm-Charles J. Fickey [57] ABSTRACT Method for providing electroluminescent emission in an electrolytic cell by subjecting to an alternating current an electrolytic cell containing at least two electrodes in a mixture of (a) a substantially inert solvent, (b) a fluorescent organic polycyclic heterocyclic compound, and a supporting electrolyte.

7 Claims, N0 Drawings METHOD FOR PROVIDING ELECTROLUMINESCENT LIGHT FROM A POLYCYCLIC HETEROCYCLIC COMPOUND IN AN APROTIC SOLVENT UNDER ELECTRIC CURRENT This is a division, of application Ser. No. 504,1 ll filed Oct. 23, 1965, now US. Pat. No. 3,654,525, which, in turn, is a continuation-in-part of Ser. No. 382,408, filed July 13, 1964, and now abandoned.

The present invention relates to solution phase electroluminescence. It has been found, pursuant to the instant discovery, that visible electroluminescent emission may be generated by applying alternating current, at a sufficient voltage, to the electrodes, e.g., platinum, mercury, or the like, of an electrolytic cell in an inert solvent containing a fluorescent organic compound and a suitable supporting electrolyte.

Typically, dimethylformamide (DMF) solvent containing 2 X mole of rubrene as the fluorescent compound and 0.1 mole of tetrabutylammonium perchlorate as the supporting electrolyte is a system which will emit light, without any appreciable consumption of the solution components of the system as compared to prior systems, when placed in an electrolytic cell containing electrodes and 60-cycle alternating current applied to the electrodes. Visible light is emitted at or near each electrode surface as long as alternating current of sufficeint voltage is applied.

Pursuant to the instant discovery, therefore, a method of generating a useful, visible, electroluminescent emission in an electrolytic cell has been found which comprises subjecting an electrolytic cell containing at least two electrodes in a medium comprising an inert solvent, a fluorescent organic compound, and a supporting electrolyte to an alternating current at a sufficient voltage (potential) at at least one electrode to convert said fluorescent organic compound to its corresponding oxidized or reduced state, by giving up or taking on at least one electron, and said alternating current providing sufficient potential change on reversal of the alternating cycle to provide an amount of energy about sufficient to ultimately transform (regenerate) said fluorescent organic compound to its original oxidation state but in its singlet excited state. The compound rapidly returns to its ground (non-excited) state by the emission of light.

As just indicated, the fluorescent organic compound is either alternately oxidized to an oxidized state (i.e., a cation radical) in what is the anodic excursion of the applied potential and reduced to the excited state of the fluorescent organic compound in what is the cathodic excursion of the applied potential; or the fluo' rescent organic compound is reduced to a reduced state (i.e., an anion radical) in what is the cathodic excursion of the applied potential and oxidized to the excited state of the fluorescent organic compound in what is the anodic excursion of the applied potential. Fluorescent compounds which emit red light upon excitation require the least anodic or cathodic voltage excursion and, consequently, the least voltage change at an electrode to provide visible light. On the other hand, fluorescent compounds which emit blue light upon excitation require greater anodic or cathodic excursions and higher voltage change at the electrode.

The upper and lower limits of the instantaneous potential applied to the electrode required to produce light will depend on the fluorescent organic compound used. Thus either the upper limit of the applied potential must be sufficiently positiveto convert the fluorescent organic compound to an oxidized state or the lower limit of the potential applied to the electrode must be sufficiently negative to convert the fluorescent organic compound to a reduced state. Moreover, the potential difference between the upper and lower values of theinstantaneous applied potential must be at least about sufficient to provide enough energy to produce said fluorescent organic compound in its singlet excited state.

In general terms, the process described above requires only electron transfer to a cation radical or electron transfer from an anion radical in an electrolyte cell where electron transfer occurs over a sufficient potential to provide an excited state, and where the resulting excited state or a subsequently formed excited state is capable of fluorescent emission of light. The general process is described in equation (1) and (2) below where A and A refer to a cation radical and an anion radical respectively, E refers to an electron, and A refers to an excited state produced by electron transfer.

The singlet excited state of the fluorescent organic molecule may be obtained directly typically as in equation (3) or by an indirect route, typically as shown in equation (4) below.

The potential difference required by the indirect route normally is lower than that required by the direct route.

Generally, the potential difference between the upper and lower limits of the instantaneous applied voltage must exceed about 1.5 volts.

Potentials (relative to a standard electrode, such as the saturated calomel electrode,) required to oxidize or reduce organic compounds of the type contemplated herein can be easily measured by standard polarographic techniques. Cf. I. M. Kolthoff and J. J. Lingane, Polarograph, 2nd Edition, 1952, lnterscience Publishing, New York, N. Y. Likewise, minimum energy required for converting organic compounds of the type contemplated herein to their singlet excited states are easily measured by such techniques as absorption or emission spectroscopy. Cf. S. F. Mason, Molecular Electronic Absorption Spectra, Quarterly Reviews, 15, 287 (1961).

The process of the present invention has, as will be seen hereinafter, multiple uses in the fields of illumina tion, information display, etc. For instance, the electrolytic cell of Example 1, infra, is in essence a light bulb, the electrolytic cell comprising a stoppered transparent bottole having two electrodes therein, the ends of which are immersed in the fluorescent-solventelectrolyte system. If desired, the bulb-shaped cell could be relaced by a tubular, or cube-shaped cell, or by any other design desired. Likewise, multiple pairs of electrodes may be used in any given cell, each pair operating independently, if desired. Still other uses will be discussed in greater detail hereinafter.

Obviously, as indicated hereinabove, the solution system as well as the nature of the electrode determine the upper limit of the potential difference. Insofar as the frequency of the applied alternating voltage is concerned, it can range from a few cycles per minute up through the audio range and beyond.

Numerous organic fluorescent compounds are capable of yielding visible light when alternating current, at a sufficient voltage, is applied thereto pursuant to the instant discovery. Typical of these are fluorescent aromatic polycyclic hydrocarbons and polycyclic heterocyclic fluorescent compounds. Among these compounds are: 1,4-dimethoxy-antracene; l-methoxy- 9, l O-diphenyl anthracene; l ,4-dimethoxy-9,10- diphenyl anthracene; 2,3-benzofluoranthrene; anthracene; rubrene; pyrene; coronene, decacylene; B-dinaphthylene oxide; l,3-diphenylisobenzofuran (i.e., 2,7-diphenylisobenzofuran--- 1,7-phenanthroline; benzo-(l,2,3-g,h: 4,5,6-g',h)- diphenanthridine; N-methyl phenothiazine; and the like.

Other examples among these above typical compounds are: 2,7-bisdimethylaminonaphthalene (gives 3,4,6,7-tetraphenylisobenzofuran, and 2,3,4,5,6,7-hexaphenylisobenzofuran 7,7,8,8-tetraphenylquinodimethane; 9,10- diphenylanthracene; l,4,5,8-tetraphenylnaphthalene; 7,7,8,8-tetraphenylquino-dimethan l ,4-diphenyl-l ,3-butadiene; l,l ,4,4-tetraphenylbutadiene; quarterphenyl; benzophenanthranthrene (i.e., chrysene); naphthacene; coronene; decacyclene; l,2,5,6-dibenzanthracene (i.e., dinaphthanthracene); etc.

A wide variety of supporting electrolytes may be employed herein to effect the invention. It is essential that these electrolytes do not hinder to any substantial degree the necessary anodic or cathodic excursion, for instance, and thus prevent conversion of the organic fluorescent compound to its excited state. It will be recognized by the person skilled in the art that a noninterfering electrolyte for one organic fluorescent compound may interfere with another organic fluorescent compound, and vice versa. Obviously, therefore, it is within the purview of the instant discovery and within the skill of a chemist to employ an electrolyte which is compatible with an organic fluorescent compound employed. The electrolyte should likewise be electroinactiveover the potential range required for the luminescent reaction, it should provide satisfactory conductivity, and it should not quench the luminescence.

Typical suitable electrolytic cations are tetralkyl (lower) ammonium ions, alkali metal ions, alkaline earth ions, and the like. Typical anions are perchlorate ions, hexafluoroarsenate ions, hexafluoraphosphate ions, chloride ions, bromide ions, and the like. Thus, typical compounds include tetraethylammonium bromide, tetraethyl-ammonium perchlorate, tetra-n-butyl ammonium perchlorate, lithium bromide, sodium perchlorate, tetramethylammonium hexafluoroarsenate, tetrabutylammonium tetraphenyl borate, calcium perchlorate, tetrapropyl ammonium hexafluorophosphate, lithium aluminum chloride, tetrabutyl ammonium bromide, etc.

Insofar as solvents are concerned, a wide variety of these may be employed. In fact, any substantially inert organic or inorganic solvent for the organic fluorescent compound and electrolyte, which solvent is sufficiently non-protonating and irreducible to preserve the desired degree of reversibility (i.e., it should provide a lifetime of the radical ion at least equivalent to the reciprocal of the frequency employed) is satisfactory provided it is rendered conducting by the addition of an electrolyte of the type contemplated herein.

Typical solvents are the following aprotic solvents: nitriles, such as acetonitrile, sulfoxides, such as dimethylsulfoxide; ethers, such as tetrahydrofuran, dioxane, diethyl ether, dimethoxyethane, and the like; amides, such as dimethylformamide; carbonates, such as propylene carbonate; nitroalkanes, such as nitromethane; dialkyl sulfites, such as dimethylsulfite; and other like solvents.

It is not necessary that these solvents be anhydrous, since up to about lOpercent water has been present in some cases without interfering with the emission of visible light. The person skilled in the art will recognize that numerous other substantially inert organic and in- CrHs 3,816,795,v 6- organic solvents, even though not essentially or sub- (through which nitorgen has been bubbled to substanstantially aprotic, are compatible with theprocess and tially deaerate same) are introduced sufficient tetsolution system and. are substantially not fluorescence rabutylammonium perchlorate and rubrene to provide.

quenchers. Solvent mixtures may likewise be emrespectively. a 0.1 molar solution of the electrolyte and ployed. 5 a 2 X molar solution of the organic fluorescent In conjunction with the excited state referred to h r compound. Two platinum electrodes each having one inbefore, it should be noted that theenergy of an exsquare inch platinum grids on the ends thereof are cited state is an easily measured experimental value. passed through the pp into the bottle, the g For example, the energy difference betweena fi t xends, of course, being immersed in the solution. These cited singlet and its corresponding ground stat i d 10 electrodes, in their vertical positions, are spaced about fined by the frequency f the f r t ab ti b d i one-half inch apart and alternating current (60 cycle) the ultra-violet or visible spectrum of the ground state is Passed therethfough at three VOltS- The grids light P species. andthis visible light is evident so long as the current re- The physical energy released by a reaction is also an mains experimental quantity. For instance, the energy of a reaction-of the type given in the specific embodiment de- EXAMPLE Vm scribed above can be determined by polarographic Example I is repeated in every essential respect with measurements and other procedures well known to the the exception that the following fluorescent comphysical chemist. pounds, electrolytes and solvents are employed at the Thus the operable limits of electroluminescence are congentjtionigivini H g H v capable of independent measurement and of clear defi- As in Example I, supra, the grids light up and visible nition in terms of physical characteristics. Conselight is evident in each of these examples so long as the quently, generating electroluminescent emission by the current is on.

Molar concentration Organic fluorescent Example number compound (a) Electrolyte (b) (a) (b) Solvent (0) II Rubrene Lithium bromide 2 10- 0 1 DMF.

d Tetrabutylammonium perchlorate- 2X10-3 0 1 95% dioxane, 5% Hi0. taxio- 0.1 DMF. racene 2X10-9 0.1 DMF. VL. 9,10-diphenyl anthracene. 2 10- 0.1 DMF. VII N-methyl phenothiazine- 2 1O- 0 1 DMF. VIII 9,10-diphenyl anthracene. 2X10 0 1 Glyme(1,2-dimethoxyethane).

process contemplated herein can be accomplished by EXAMPLE IX first recognizing the known physical characteristics of the fluorescent organic compound, as well as the physical characteristics of the inert solvent and the electro lyte to be used. It has been found, however, that the potential change during the electrode excursion can be several tenths of a volt less than that required to provide the energy of a singlet excited state and still be sufficient to generate noticeable light emission. Best re- Example 1, above, is repeated in every essential respect with the exception that carbon is used in lieu of platinum on the electrode. Visible light is emitted when the current is applied.

sults are generally obtained, however, when the calcu- EXAMPLE X lated singlet excitation energy or more is provided, I I

Should also be noted that the voltages referred to are Example 1, above, is r peated in every essential reexclusive of additional Yoltages that might be required spect with the exception that stainless steel is used in to overcome the electrical resistance of the solventlieu of platinum on the electrode sible light is emitelectrolyte employed 5 ted when the current is applied.

The temperature at which the process of the present 5 invention is carried out is not critical; very excellent results have been achieved at ambient temperatures. For EXAMPLE XI best results the solvent employed is deaerated, such as by bubbling nitrogen, or the like, therethrough, thus providing improved conditions and helping to insure a EXamPle above, is repeated in every essential b i ll inert l spect with the exception that tantalum is used in lieu of platinum of the electrode. Visible light is emitted when the current is applied.

EXAMPLE I The following examples further illustrate the present lnto a SO-milliliter, rubber-stoppered bottle containinvention, all of which are carried out essentially as in ing 30 milliliters of dimethylformamide (DMF) solvent Example I, supra, excepting as shown.

Molar concentration Alternating current Example Organic fluorescent number compound (a) Electroylte (b) (a) (b) Solvent Frequency Volts III Rubreno Tetrabutyl-ammonium per- 2X10 0. 1 DMF 0 3 2. 5

chlorate do o... 0.1 DMF- 30 5 do do. 0.1 DMF 100 XV 2,3-benzolluoranthrene Sodium perchlorate..- 2X10 0. 1 Glyme... 60 20 XVI Dlnaphthalene oxlde Teltramethyl arnmonium hexa- 2Xl0- O. 1 Propylene carbonate... 60 5 UOIO BISQUE. 0. XVII 1,3diphenylisobcnzoturan... Tetgabultyg amtmonlum tetra- 3X16' 0. 1 Tetrahydroturan 6O 20 p on ore e. XVIII 1,7-phenanthroline Ca(Cl 10 0.1 Dimethylsulfoxide 6O 7 XIX 1,10-phenanthroline LiClO 4X10- 0.1 Ethylene diamine 60 XX Benzo-(1,2,3-g,h:4,5,6,gh)d.iphen- Tetrahutyl-ammonium per- 2X10 0. 1 Glyme 6O 10 anthradtne. c orate. XXI N-methyl phenothiazine Tetraprofiyl ammonium hexa- 8X10- 0. 1 Propylene carbonate.- 60 10 fiuorop osphate. XXII 9,10-diphenylanthracene Tetlrlabuttylammonium per- 10- 0.8 Glyme 6O 15 c ora e 5X10 0. 60 12 1Xl0- 0. 60 10 1Xl0- 0. 60 10- 1. 6O 10 oene. XXVII lAfilimethoxy-Q,IO-diphenylan- Tetrabutylammonium bromide... 10 0. 6O 7 t acene. XXVIII. Pyrene MgClOi 2X10" 0. 20 15 XXIX Coronene Tetrabutylammonium per- 10- 0. 60 20 chlorate. XXX Decaeyclene do 10- 0. 60 20 XXXI 1,4-dimethoxyanthraeene Sodium perchlorate 10- 0. 60 10 Cycles per second. In square waves (peak to peak). m 'Iotal volts applied to cell.

The examples in the above table produce a visible electroluminescent emission. In Examples I through XI, and XIII through XXXI, above, voltage is varied with respect to time as a sine wave function.

While the above table shows frequencies on the order 0.3 to 100 cycles per second, substantially higher frequencies as indicated hereinbefore may be employed. For instance, the frequencies of Examples XIII, XVII, XXI and XXVI may be changed to 500, 1000, 10,000 and 15,000, respectively, without materially modifying the electroluminescent emission.

Uses for the present invention other than those discussed hereinabove involve establishing spaced apart plates, screens or similar surfaces which act as electrodes; one or more of said surfaces may be meshed, perforated or translucent, if desired. Between these surfaces the fiuorescentsolvent-electrolyte system, preferably in a gel form, is placed and alternating current passed through the electrodes and, of course, the intervening system. Upon illumination of the electrodes visible light can be seen.

One or both of these electrodes could contain information, such as that provided by various perforations. The electrodes, should they be solid, translucent sheets, could be covered by a stencil containing information, thus only the light passing through the stencil being visible. Obviously, multiple individual electrode pairs, the electrodes being of the type just described, could be used, allowing greater flexibility in their use as information displays.

According to still another use embodiment of the present invention, a multiplicity of electrodes of any shape (e.g., straight rods, curved rods, tapered rods, etc.) may be placed on a panel and the electrodes immersed in a system (gel or solution) of the type contemplated herein, the electrodes being connected in pairs, or in any other suitable manner, to the alternating current. By controlling the current on each of these pairs various information could be produced on the panel.

As to the embodiments referred to hereinabove wherein the fluorescent-solution-electrolyte system is a gel, the following examples are typical:

EXAMPLE XXXII Example XIII, supra, is repeated in every essential respect with the exception that 30 percent by weight of polyacrylamide is dissolved in the solvent. A gel results which may be used as above indicated, such as between two electrode plates in sandwich-like fashion.

EXAMPLE XXXIII A solution of 10 percent acrylamide, 10 percent methylene-bisacrylamide, 0.5 percent a,oz-azobisisobutyronitrile, 5X10 molar rubrene, 0.1 molar tetrabutylammonium perchlorate in dimethylformamide in which is immersed two platinum electrodes is warmed at C. for several hours until polymerization is essentially complete. A semisolid gel is obtained. Application of 60-cycle alternating current at five volts provides a yellow light.

If desired, the electrolytic cell container, translucent or transparent, may be coated (e.g., exteriorly) with a fluorescent material and light emitted by the electrodes converted by means of this material to a light of a longer wavelength. The following examples are illustrative:

EXAMPLE XXXIV Example VI, above, is carried out in a glass container coated on outside with a five millimeter thick film of polystyrene containing 1 percent Rhodamine B. Red light (rather than blue) is emitted on application of alternating current.

EXAMPLE XXXV Example XXXIV is repeated only 1 percent eosin is used in lieu of Rhodamine B.

Numerous modifications will be evident from the fluorescent art. Thus, instead of an organic fluorescent dye (as in Examples XXXlV and XXXV), a crystal phosphor may be employed in the coating. Cf. P. Pringsheim, Fluorescence lnterscience Publishers, New York, 1949, Chaper Vll.

Clearly the instant discovery encompasses numerous modifications within the skill of the art. Consequently, while the present invention has been described in detail with respect to specific embodiments thereof, it is not intended that these details be construed as limitations upon the scope of the invention, except insofar as they appear in the appended claims.

We claim:

1. A method of generating a useful, visible, electroluminescent emission in an electrolytic cell which comprises subjecting to an alternating current an electrolytic cell containing at least two electrodes in a medium comprising (a) a substantially inert solvent, (b) a fluorescent organic polycyclic heterocyclic compound, and (c) a supporting electrolyte, said alternating current and Ph osphorescence,

being at a sufficient voltage (potential) in at least one electrode to convert said fluorescent organic compound to its corresponding oxidized or reduced state, by giving up or taking on at least one electron, and said alternating current providing sufficient potential (voltage) change on reversal of the alternating cycle to provide an amount of energy about sufficient to substantially regenerate said fluorescent organic compound in its singlet excited state.

2. The process of claim wherein the fluorescent compound is N-methyl phenothiazine.

3. The process of claim 1 wherein the electrolyte is tetrabutylammonium perchlorate.

4. The process of claim 1 wherein the electrolyte is lithium bromide.

5. The process of claim 1 wherein the solvent is dimethylformamide.

6. The process of claim 1 wherein the solvent is dioxane containing up to about 10 per cent water.

7. The process of claim 1 wherein the solvent is 1,2-

dimethoxyethane. 

2. The process of claim wherein the fluorescent compound is N-methyl phenothiazine.
 3. The process of claim 1 wherein the electrolyte is tetrabutylammonium perchlorate.
 4. The process of claim 1 wherein the electrolyte is lithium bromide.
 5. The process of claim 1 wherein the solvent is dimethylformamide.
 6. The process of claim 1 wherein the solvent is dioxane containing up to about 10 per cent water.
 7. The process of claim 1 wherein the solvent is 1,2-dimethoxyethane. 